PRIEST VALLEY — A morning breeze carries the sounds of gentle grunts and subdued squeals across a shallow valley bathed in rosy light. Behind scrubby oaks, a hunting guide and two clients inhale the earthy, maple musk wafting from more than two dozen wild pigs tearing the hillside apart in search of food after a long night of foraging. One hunter aims his rifle, exhales and squeezes the trigger before — POP — the panicked pack scatters.

The hunters can only watch.

Guide Tony Phelps, co-owner of All Season Outfitters, leads Fremont resident Paul Keswick and his 19-year-old daughter, Kiley, through the rolling hills of a Priest Valley ranch to track and kill wild pigs. Clients come from all over the state to hunt in the heart of pig country.

Pig hunters play an important role in California”s efforts to control destructive wild pig populations that threaten the state”s native wildlife, agriculture and public health. The animals dig up native plants, destroy farmers” fields, compete for resources with other animals and reproduce at an astronomical rate. The pigs have become such a nuisance in the Almaden Valley and Coyote Creek areas of San Jose that the City Council last week voted to make permanent a trial law that allows licensed trappers to catch and shoot boars within the city limits.

“Pigs have exploded in this state,” said Craig Stowers, environmental program manager for the California Department of Fish and Wildlife. “If they get a foothold somewhere, they”re going to take off. And that”s what they”ve done.”

In Monterey County, wild pigs thrive in the oak-dominated landscapes and grasslands — the perfect habitat for the animal”s lifestyle.

California”s wild pigs are an invasive species. They first appeared in the 1700s when European explorers let their domestic hogs forage freely for fallen acorns. In the 1920s, wealthy Englishmen who lived in Monterey County brought over the European wild boar to continue the hunting tradition they had enjoyed across the pond. In both cases, the swine broke free from their pens and went feral.

Many hunters use “pig” and “boar” interchangeably. Indeed, all pigs have a common ancestor in the European wild boar. But pigs are slightly smaller than the pure wild boar — the average adult pig weighs in at 150 to 200 pounds, while the average adult boar is between 200 and 300 pounds. After a few generations of living in the wild, however, the offspring have become more boarlike as feral hogs and boars interbreed.

By the 1950s, California”s pig population had skyrocketed and the public pressured the state Fish and Game Commission to officially designate pigs as a game species, said Reginald Barrett, a UC Berkeley wildlife biologist who has studied wild pig populations and management strategies in California for four decades.

Hunters moved the wild boar from Monterey County to other parts of the state to increase their hunting opportunities. Now the pigs live in 56 of California”s 58 counties, and they number between 200,000 and 1 million.

As a result, the state”s pig hunting regulations are extremely relaxed. Hunters still need licenses and must buy tags for each pig they want to shoot, but there are no limits to the number of pigs someone can bag. And the season is open year-round. “The idea is to keep the pig population controlled,” Barrett said.

Landowners can cash in by letting hunters pay to kill on their land. Or if pigs are destroying their property, they can apply for a Fish and Game permit to take care of the pests themselves. California law allows property owners to shoot a destructive pig on sight without a permit as long they report it within 24 hours.

“The reality is there are a lot of property owners who deal with the pigs without telling anybody,” Barrett said. “It”s called the shoot, shovel and shut-up policy.”

The animals can eat almost anything, including amphibians, reptiles and their eggs. They forage by digging with their noses, leaving a destructive path that pig hunters follow to stalk their kill.

“Tracks, rooting, rubbing up on trees — you can tell the size of the boar by where the teeth hit up on the trees,” said Phelps, explaining how he guides his clients to a wild pig pack.

But the behavior can damage oak trees and other native plants. Monterey County has a high density of native oak trees, which are particularly sensitive to wild pig foraging. When the oaks drop their acorns in the fall, the pigs snatch them up, quickly eliminating the ability of oak stands to reproduce. Additionally, the wild pigs compete for the same resources as native wildlife species — and they don”t like to share.

“When the pigs move in, you can bet the other animals are moving out,” Fish and Wildlife”s Stowers said. “They just tear up (the habitat) and make it unusable for anything else. They”re pigs. They call them that for a reason.”

The animals are also a big problem for ranchers and farmers. The 2006 E. coli outbreak that killed three people and sickened more than 200 most likely started from spinach leaves contaminated with feces from wild pigs or cattle at a San Benito County ranch, federal and state investigators concluded .

Even with such relaxed hunting regulations in place, limiting the pigs” population growth is difficult, something sport hunters like Paul Keswick, the Fremont resident, are well aware of. A sow can have two litters a year with about six piglets each. “Some don”t make it due to predators and such, but that”s still growing at a faster rate than we hunters can take out,” Keswick said.

Another reason that hunting falls short as a management strategy is the same reason wild hogs are thrilling to hunt — they are cunning, intelligent and are often several hooves ahead of the hunters.

“Pigs are one of the smartest animals on the face of the earth. They are very adept at surviving,” Stowers said. “They”ll move out of an area, and they”ll keep moving until they don”t have any pressure.”

At Pinnacles National Park, the pigs” damage to the federally protected landscape required more than just hunting. The park constructed about 24 miles of fence to protect 14,000 acres, said Daniel Ryan, the park”s invasive-wildlife biologist.

The fence has been a success, park officials say. Since its completion in 2003, no pigs have broken in, and the damaged vegetation has regrown quickly. The construction and maintenance of the fence cost about $2 million, but Ryan said the short-term investment could pay off in reducing higher long-term costs associated with pig control.

“The other factor that has to be considered is the value of the resources you want to protect. Quantifying the value of a natural environment is a notoriously difficult thing to do. How much is an intact oak woodland worth — $5,000, $50,000, $500,000, $5 million?” he said. “The cost of the fence may be cheap in comparison to the value of what you want to protect.”

The park in 2010 added eight miles more fence to protect an additional 500 acres.

Though hunters can”t eliminate the problem, experts say, they are an important part of an evolving strategy.

“I still think hunting will play a role in managing these populations,” Stowers said. “But it”s gone beyond the ability of any normal hunting program.”